In high-frequency transmission lines used in, for example, packages for high-frequency semiconductor elements or wiring boards for circuit element mounting, the mounting positions of electronic components, surface lines that are formed on the surface of a dielectric wiring board, and internal lines that are formed in the interior of a dielectric wiring board are frequently interconnected.
Representative examples of surface lines that are formed on the surface of a dielectric wiring board include microstrip lines and coplanar lines. In addition, representative examples of inner-layer lines that are formed inside a dielectric wiring board include strip lines and coplanar lines. Further, regarding interconnections between surface lines and inner-layer lines, connections are realized by vias or through-holes having conductivity.
As an example, the high-frequency wiring board described in JP-A-2003-133472 (hereinbelow referred to as Patent Document 1) has high-frequency transmission lines as shown in FIGS. 1A-1D. FIG. 1A is an overall perspective view of the high-frequency wiring board, FIG. 1B is a perspective view of the second dielectric layer portion of the high-frequency wiring board, FIG. 1C is an upper plan view of the reverse-surface conductive pattern of the high-frequency wiring board, and FIG. 1D is a sectional view taken along line X-X in the direction of signal transmission of the high-frequency wiring board shown in FIG. 1A.
The high-frequency wiring board shown in these figures is composed of dielectric wiring board 20 (FIG. 1A) realized by stacking two dielectric layers 20a (FIG. 1A) and 20b (FIGS. 1A and 1B). High-frequency transmission lines are then formed on different layers.
The first high-frequency transmission lines are made up from: first signal lines 10 (FIG. 1A) formed on the upper surface of first dielectric layer 20a (FIG. 1A) that is the obverse surface of dielectric wiring board 20 (FIG. 1A), first ground pattern 30 (FIGS. 1A and 1D) that is arranged around these signal lines 10 (FIG. 1A) and on the same surface, and second ground pattern 32 (FIGS. 1A, 1B, and 1D) formed on the surface of second dielectric layer 20b (FIGS. 1A and 1B). In addition, second high-frequency transmission lines are made up from the above-described first ground pattern 30 (FIGS. 1A and 1D), third ground pattern 31 (FIGS. 1A and 1C) formed on the lower surface of second dielectric layer 20b (FIGS. 1A and 1B) that is the reverse surface of dielectric wiring board 20 (FIG. 1A), second signal line 11 (FIG. 1B) formed on the upper surface of second dielectric layer 20b (FIGS. 1A and 1B) and arranged between these ground patterns, and second ground pattern 32 (FIGS. 1A, 1B, and 1D) that is arranged around this signal line 11 (FIG. 1B) and on the same surface.
The end of first signal line 10 (FIG. 1A) of the first high-frequency transmission lines and the end of second signal line 11 (FIG. 1B) of the second high-frequency transmission lines are connected by via 40 (FIG. 1A) having conductivity. In addition, first ground pattern 30 (FIGS. 1A and 1D), second ground pattern 32 (FIGS. 1A, 1B, and 1D), and third ground pattern 31 (FIGS. 1A and 1C) are electrically connected by a plurality of conductive vias 41 arranged along the signal transmission direction of first signal lines 10 (FIG. 1A) and second signal line 11 (FIG. 1B).
However, when different line constructions are connected together, as with first high-frequency transmission lines and second high-frequency transmission lines, mismatching tends to occur in the vicinity of the connections, and as a result, signal reflection tends to occur increasingly as the frequency of signals increases.
As a result, methods have been proposed as in, for example, JP-A-2004-320109 (hereinbelow referred to as Patent Document 2) for limiting impedance mismatching and thus decreasing signal reflection by changing the end width of signal lines that correspond to first signal lines 10 (FIG. 1A) that make up the above-described first high-frequency transmission lines, i.e., changing the width in the vicinity of connections with conductive vias 40 (FIG. 1A).
Patent Document 1: JP-A-2003-133472 (FIG. 5)
Patent Document 2: JP-A-2004-320109 (FIG. 1, paragraph 0095)
As described hereinabove, when connecting signal lines of different types in which signal lines are formed on different layers in the configuration shown in FIGS. 1A-1D, changing the signal line width in the vicinities of conductive vias that interconnect signal lines results in an improvement of the signal pass characteristic (also called reflection characteristics). However, it was found that the problem in which the signal pass characteristic (also called the reflection characteristics) deteriorated as the transmission signal went from a low frequency to higher frequencies could not be solved in this related art.
The reasons for this problem are next explained with reference to FIG. 1D.
In the configuration shown by FIGS. 1A-1D, when a signal is transmitted from the first high-frequency transmission lines to the second high-frequency transmission lines, the signal-line component of the current among the high-frequency current that is propagated over first ground pattern 30 (FIGS. 1A and 1D) and first signal line 10 (FIG. 1A) of the first high-frequency transmission lines flows along second signal line 11 (FIG. 1B) of the second high-frequency transmission lines. However, the ground-pattern component of the current not only flows through second ground pattern 32 (FIGS. 1A, 1B, and 1D) of the second high-frequency transmission lines but also through first ground pattern 30 (FIGS. 1A and 1D), i.e., in two paths. In other words, as shown in FIG. 1D, the current is propagated on path A that passes by only first ground pattern 30 (FIGS. 1A and 1D) and on path B that passes successively from first ground pattern 30 (FIGS. 1A and 1D) to conductive via 41a (FIG. 1D), second ground pattern 32 (FIGS. 1A, 1B, and 1D), and the next conductive via 41b (FIG. 1D) along the signal transmission direction before again returning to first ground pattern 30 (FIGS. 1A and 1D).
If the physical path lengths of paths A and B are L1 and L2, respectively as shown in FIG. 1D, then the path length difference L1-L2 is ΔL, the wavelength of signal transmission in a vacuum is λ0, the wave number of each path is the same at k, and the effective relative dielectric constants on each path are the same at ∈, the phase difference between the two paths A and B is represented by:
      [          Formula      ⁢                          ⁢      1        ]                                                          k              ×              L              ⁢                                                          ⁢              1                        -                          k              ×              L              ⁢                                                          ⁢              2                                =                                    k              ×              Δ              ⁢                                                          ⁢              L                        =                                                            (                                                            2                      ⁢                      π                                                              λ                      ⁢                                                                                          ⁢                                              0                        /                                                  ɛ                                                                                                      )                                ×                Δ                ⁢                                                                  ⁢                L                            =                                                (                                      2                    ⁢                    π                    ×                                          ɛ                                                        )                                ×                                  (                                                            Δ                      ⁢                                                                                          ⁢                      L                                                              λ                      ⁢                                                                                          ⁢                      0                                                        )                                                                                          (          1          )                    and is proportional to ΔL/λ0.
As a result, even if the physical path length difference ΔL is fixed, interpath phase difference tends to increase and phase difference more readily occurs as the transmission signal progresses from a low frequency to a higher frequency, i.e., with shorter wavelength of wavelength λ0.
Essentially, it was found that even when adopting the method taught in Patent Document 2, the potential for improving the reflection characteristics of signal transmission from the first high-frequency transmission lines to the second high-frequency transmission lines in the configuration shown in FIGS. 1A-1D diminishes with higher frequencies.